I live in the southeast of England where the present land use is 50% grass, 30% B&Q car parks and the rest is just the usual urban rubbish.
Thousands of years ago there was probably, (I cant be bothered to do the research), there was a lot more grass.
A cousin who is keen on geneology tells me that our family name is Roman\British so that means we’ve been in this neck of the grass for 60 generations at least.
Now I know that hay fever is hereditary because the family has been meeting up to watch the world cup and most have puffy, slitty eyes and keep SNEEZING!!. It looks like a meeting of the Mongolian politburo 
and there are tiny rainbow dots on the TV screen.
My question is - how did this hereditary disorder survive to be passed down to ME? Years ago , on high pollen days, we must have been a pushover to grass-immune invaders, and the sneeze- through-June gene should have been eliminated.
'Cause Hayfever rarely kills people before they can pass their genes on to the next generation.
I also suffer from hay fever at various times of the year. I have found neither prescription nor over-the-counter drugs to be very helpful. However, I have found that wearing a (cheap) paper mask (made by 3M) over mouth and nose clears it up within 10 minutes. But I rarely wear such mask in public because of the wierd stares. Some people can’t tolerate this mask because you have to breath deeper or get stale air.
Most English surnames originate from the fifteenth century or later. Those that are older are usually French in origin. Some English place names do date from Roman times and adopting a place name is one of the most common family naming methods.
Your ancestors may have been British for several millenia but ragweed hasn’t. The plant responsible for hayfever is native to North America.
Whatever is prompting pluralgravity’s misery, it’s not ragweed - that doesn’t pollinate until August at the earliest. It probably is grass.
Prior to the 19th Century allergies of any sort were almost unheard of, and rare through the mid-20th. The rate of allergies increases with better hygience and industrialization, one theory being that keeping kids too clean contributes to this immune system dysfunction. Truthfully, no one knows for sure, except that more and more people are becoming allergic.
It may even have the opposite effect; “aww, poor you, let me see if I can cheer you up…”… maybe not.
Anyway, I think thousands of years ago, much of Britain would have been wooded, rather than vast expanses of grass - if you don’t believe me try not mowing your lawn for fifteen years; you’ll have woodland instead.